Normally, I would agree with what everyone said and go to Epcot on your first day. This is usually what we always do. But this past January I took my mother (who had never been to WDW and was celebrating her 80th birthday but is very spry, etc. etc.), and we arrived at the Poly and checked in and our room was ready at 12 noon. We dropped our luggage and Mom asked to go to the MK. Well, I had already planned to do this but was a bit worried about crowds since it was Martin Luther King Day and only 1 of the 2 days during the week that Spectromagic was showing! I could NOT refuse my mom and figured even if we just enjoyed the atmosphere and had our dinner at LTT and saw Spectro, she would be happy.
Well, we hit Main Street at around 1 pm and it was PACKED. Mom didn't even notice, but I was a bit panic stricken. Well, she was the one who calmed me down when she saw Pinocchio and Gepetto and demanded to have her picture taken with them. I knew then that this was going to be a GREAT afternoon and evening. So we went with the flow and followed my plan to head over to Adventureland and do the attractions there. It was GREAT!!!! We got a FP for the Jungle Cruise, did the Tiki Birds, and then POC, then used our FP's for the Jungle Cruise. My DS 12 then did those remote control boats outside of the Jungle Cruise, while I went to get Mickey bars for all of us. After we ate them, I noticed that the Magic Carpets of Aladdin had NO line, and Mom wanted to go on them twice so we did! When that was done, we headed over to LTT for our character dinner. We got great curbside "seats" for Spectro and then headed back to the Poly (to avoid the after Wishes crowd).
So it is possible to do MK on a busy afternoon, but you have to set your expectations correctly.